1. Field of the Invention
The present invention relates to a load cell with a base plate, and more particularly, to a load cell with a base plate integratedly fixed to a load support section and able to detect the magnitude of a load in terms of a quantity of electricity.
2. Description of the Prior Art
Various configurations of a strain sensing section of a load cell have been proposed to meet configurations and accuracy requirements, such as a small-size or thin-shape load cell, and are in practical use.
The most popular type of load cell used for measuring a heavy load has a strain gauge attached to a shear beam fixed at both ends, as this effectively prevents movement of a load application point.
FIG. 6 is a plan view of a load cell and FIG. 7 is a cross sectional view taken along the line 7--7 of FIG. 6, and these figures respectively depict the construction of major sections of a prior art load cell comprising a shear beam fixed at both ends.
As shown in the figures, a load cell 1 comprises a cylindrical outer shell section 2 for supporting a load, a load seat 3 installed at the center of the outer shell section 2 to accomodate a load to be measured, a load transmission section 4 formed coaxially and as one body with the load seat 3 for transmitting a load accomodated on the load seat 3 to a strain sensing section, described later, four cross-shaped shear beams 5, 6, 7, and 8 connected to the outer periphery of the load transmission section 4 and the inner periphery of the outer shell section 2. The middle sections of the shear beams 5 to 8 serve as strain sensing sections 5a, 6a, 7a, and 8a respectively, and a strain gauge SG is attached to each of the strain sensing section 5a to 8a in parallel to the load axes thereof respectively, for detecting a shear strain. More specifically, the respective strain gauges SG are attached with the load sensing axes thereof arranged at angles of 45 deg. and 135 deg. to the load axes, respectively.
The functions of the prior art load cell 1 are described in the following. When a load W to be measured is applied to the load seat 3, the load W is transmitted to the inner ends of the shear beams 5 to 8 through the load transmission section 4, which has a greater rigidity. The reaction force, which is identical to the load W in magnitude but opposite to the load W in direction, is transmitted to the outer ends of the shear beams 5 to 8 through the outer shell section 2. Accordingly a shear force acts on the shear beams 5 to 8, to thereby generate a shear stress on the strain sensing sections 5a to 8a, respectively. The shear stress thus generated is converted into a quantity of electricity (change in electric resistance) and detected by the strain gauges SG mounted on the strain sensing sections 5a to 8a.
This prior art load cell 1 has the following drawbacks:
First, it is often impossible to maintain a required rigidity of the outer shell section 2, and therefore, the outer shell section 2 is sometimes deformed, thereby producing a moment which causes a warping of the strain sensing sections 5a to 8a, even if the load W is applied along the direction of the load axes. The errors due to the bending moment described above become a part of, or are added to the load detection output.
Namely, the load cell 1 described above, allows the generation of shear stress (bending moment) based on a normal deformation as illustrated in FIG. 8, of the strain sensing sections 5a to 8a, and can obtain a strain output equivalent to the shear stress, using the above strain gauges, when the outer shell section 2 is fixed to a solid member and thus can not move.
Nevertheless, if it is impossible to maintain a satisfactory rigidity of the outer shell section 2 due to limitations of the outer configuration thereof, when a load W is applied, a clockwise bending moment M will be produced on the load transmission section 4 side of the strain sensing section 6a, as illustrated in FIG. 9, thereby sensing both a bending stress and a shear stress on the strain sensing section 6a when a counterclockwise bending moment M' is imposed on the outer shell section 2 of the strain sensing section 6a. Since the strain generated based on the bending moments M and M' is affected by friction between an outer surface of the object being measured and the facing surface of the outer shell section 2, the strain thus generated will not be proportional to the applied load, an accurate electric measurement of the load by a Wheatstone bridge cannot be obtained and resultant errors will be added to the load detection output.
If an attempt is made to increase the rigidity of the outer shell section 2, to eliminate the above first drawback, the wall thickness of the outer shell section 2 in the load axial direction or the radial direction thereof must be increased, but if a load detection output having a greater accuracy is required, the wall thickness of the outer shell section 2 must be greatly increased, and this is obviously not a practical way of solving the above problems, due to a resulting limitation of the subject to be measured.